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Figure 1: At atmospheric pressure, dimethyl sulfoxide boils at 189 °C. In the vacuum apparatus here, it distills off into the connected receiver flask on the left at only 70 °C.

Vacuum distillation is a method of distillation performed under reduced pressure, which lowers the boiling point of most liquids.[1] As with distillation, this technique separates compounds based on differences in boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve, will cause the compound to decompose[2] or simply to save energy in heating. A reduced pressure decreases the boiling point of compounds. The reduction in boiling point can be calculated using a temperature-pressure nomograph using the Clausius–Clapeyron relation.[3]

Vacuum distillation often improves efficiency, and vacuum distillation of ocean water is considered one of the most efficient ways of desalination.

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In the laboratory vacuum distillation is used to purify compounds or remove solvent. This is generally done by applying a vacuum line to a sealed apparatus, lowering the pressure. Lowering the pressure causes most liquids to have a lower boiling point, and so less intense heat is required to boil them out of a solution. This allows for a more efficient separation process using lower temperature. To avoid the evaporated liquid entering the vacuum pump there is usually a condenser installed on the vacuum line in order to chill and condense the vapor before the vacuum pump. This allows for recovery of the liquid being removed and to avoid feeding any corrosive liquids into the vacuum pump or releasing them to the environment.[4]

Distillation is used to separate compounds based on differences in boiling point. Vacuum distillation allows for this purification technique to be used on compounds with high boiling points, or those which are air-sensitive. Compounds with a boiling point lower than 150oC can typically be distilled without reduced pressure. Using a fractionating column in the set-up improves the separation of mixtures, and can allow separation of compounds with similar boiling points. With the apparatus under reduced pressure, exposure to the atmosphere is minimised and the apparatus can be filled with inert atmosphere when the distillation is complete[5].

For better results or for very air sensitive compounds, either a Perkin triangle distillation set-up or a short-path distillation set-up can be used.

The Perkin triangle set-up (Image 5) uses a series of Teflon valves to allow the distilled fractions to be isolated from the distillation flask without the main body of the distillation set-up being removed from either the vacuum or the heat source, and thus can remain in a state of reflux.

To do this, the distillate receiver vessel is first isolated from the vacuum by means of the Teflon valves.

The vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and the distillate receiver can then be stoppered and removed from the system.

Vacuum distillation of moderately air/water-sensitive liquid can be done using standard Schlenk-line techniques. When assembling the set-up apparatus, all of the connecting lines are clamped so that they cannot pop off.

Once the apparatus is assembled, and the liquid to be distilled is in the still pot, the desired vacuum is established in the system by using the vacuum connection on the short-path distillation head. Care is taken to prevent potential "bumping" as the liquid in the still pot degases.

While establishing the vacuum, the flow of coolant is started through the short-path distillation head. Once the desired vacuum is established, heat is applied to the still pot.

If needed, the first portion of distillate can be discarded by purging with inert gas and changing out the distillate receiver.

When the distillation is complete: the heat is removed, the vacuum connection is closed, and inert gas is purged through the distillation head and the distillate receiver. While under the inert gas purge, remove the distillate receiver and cap it with an air-tight cap. The distillate receiver can be stored under vacuum or under inert gas by using the side-arm on the distillation flask.

Rotary evaporation[6] is a common technique used in laboratories to concentrate or isolate a compound from solution. Many solvents are volatile and can easily be evaporated using rotary evaporation. Even less volatile solvents can be removed by rotary evaporation under high vacuum and with heating. It is also used by environmental regulatory agencies for determining the amount of solvents in paints, coatings and inks.[7]

Safety is an important consideration when using glassware as part of the set-up. All of the glass components should be carefully examined for scratches and cracks which could result in implosions when the vacuum is applied. Wrapping as much of the glassware with tape as is practical helps to prevent dangerous scattering of glass shards in the event of an implosion.

Industrial-scale vacuum distillation[9] has several advantages. Close boiling mixtures may require many equilibrium stages to separate the key components. One tool to reduce the number of stages needed is to utilize vacuum distillation.[10] Vacuum distillation columns (as depicted in Figures 2 and 3) typically used in oil refineries have diameters ranging up to about 14 meters (46 feet), heights ranging up to about 50 meters (164 feet), and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).

Vacuum distillation increases the relative volatility of the key components in many applications. The higher the relative volatility, the more separable are the two components; this connotes fewer stages in a distillation column in order to effect the same separation between the overhead and bottoms products. Lower pressures increase relative volatilities in most systems.

A second advantage of vacuum distillation is the reduced temperature requirement at lower pressures. For many systems, the products degrade or polymerize at elevated temperatures.

Vacuum distillation can improve a separation by:

Prevention of product degradation or polymer formation because of reduced pressure leading to lower tower bottoms temperatures,

Reduction of product degradation or polymer formation because of reduced mean residence time especially in columns using packing rather than trays.

Increasing capacity, yield, and purity.

Another advantage of vacuum distillation is the reduced capital cost, at the expense of slightly more operating cost. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of a distillation column.

Vacuum distillation can also be referred to as "low-temperature distillation".

In distilling the crude oil, it is important not to subject the crude oil to temperatures above 370 to 380 °C because high molecular weight components in the crude oil will undergo thermal cracking and form petroleum coke at temperatures above that. Formation of coke would result in plugging the tubes in the furnace that heats the feed stream to the crude oil distillation column. Plugging would also occur in the piping from the furnace to the distillation column as well as in the column itself.

The constraint imposed by limiting the column inlet crude oil to a temperature of less than 370 to 380 °C yields a residual oil from the bottom of the atmospheric distillation column consisting entirely of hydrocarbons that boil above 370 to 380 °C.

To further distill the residual oil from the atmospheric distillation column, the distillation must be performed at absolute pressures as low as 10 to 40 mmHg (also referred to as Torr) so as to limit the operating temperature to less than 370 to 380 °C.

Figure 2 is a simplified process diagram of a petroleum refinery vacuum distillation column that depicts the internals of the column and Figure 3 is a photograph of a large vacuum distillation column in a petroleum refinery.

The 10 to 40 mmHg absolute pressure in a vacuum distillation column increases the volume of vapor formed per volume of liquid distilled. The result is that such columns have very large diameters.[14]

Distillation columns such those in Images 1 and 2, may have diameters of 15 meters or more, heights ranging up to about 50 meters, and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).

The vacuum distillation column internals must provide good vapor–liquid contacting while, at the same time, maintaining a very low-pressure increase from the top of the column top to the bottom. Therefore, the vacuum column uses distillation trays only where withdrawing products from the side of the column (referred to as side draws). Most of the column uses packing material for the vapor–liquid contacting because such packing has a lower pressure drop than distillation trays. This packing material can be either structured sheet metal or randomly dumped packing such as Raschig rings.

The absolute pressure of 10 to 40 mmHg in the vacuum column is most often achieved by using multiple stages of steam jet ejectors.[15]

Many industries, other than the petroleum refining industry, use vacuum distillation on a much a smaller scale. Copenhagen-based Empirical Spirits,[16] a distillery founded by former Noma chefs,[17] uses the process to create uniquely flavoured spirits. Their flagship spirit, Helena, is created using Koji, alongside Pilsner Malt and Belgian Saison Yeast[18].

Vacuum distillation is often used in large industrial plants as an efficient way to remove salt from ocean water, in order to produce fresh water. This is known as Desalination. The ocean water is placed under a vacuum to lower its boiling point and has a heat source applied, allowing the fresh water to boil off and be condensed. The condensing of the water vapor prevents the water vapor from filling the vacuum chamber, and allows the effect to run continuously without a loss of vacuum pressure. The heat removed from the water vapor is removed by a heat sink and passed into the incoming ocean water to preheat it. This reduces the energy requirement and allows for much higher efficiency due to the reduced requirement for heat and fuel usage. Some forms of distillation do not use condensers, but instead compress the vapor mechanically with a pump. This acts as a Heat pump, concentrating the heat from the vapor and allowing for the heat to be returned and reused by the incoming untreated water source. There are several forms of vacuum distillation of water, with the most common being Multiple-effect distillation, Vapor-compression desalination, and Multi-stage flash distillation.[19]

Molecular distillation is vacuum distillation below the pressure of 0.01 torr[20] (1.3 Pa). 0.01 torr is one order of magnitude above high vacuum, where fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment.[21] The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, the rate of evaporation no longer depends on pressure. That is, because the continuum assumptions of fluid dynamics no longer apply, mass transport is governed by molecular dynamics rather than fluid dynamics. Thus, a short path between the hot surface and the cold surface is necessary, typically by suspending a hot plate covered with a film of feed next to a cold plate with a line of sight in between.

Molecular distillation is used industrially for purification of oils.[22]